Although prior research has shown that experts tend to overestimate or underestimate what laypersons actually know, little is known about the specific consequences of biased estimations for communication. To investigate the impact of biased estimations of a layperson's knowledge on the effectiveness of experts' explanations, we conducted a web-based dialog experiment with 45 pairs of experts and laypersons. We manipulated the experts' mental model of the layperson by presenting them either valid information about the layperson's knowledge or information that was biased towards overestimation or underestimation. Results showed that the experts adopted the biased estimations and adapted their explanations accordingly. Consequently, the laypersons' learning from the experts' explanations was impaired when the experts overestimated or underestimated the layperson's knowledge. In addition, laypersons whose knowledge was overestimated more often generated questions that reflected comprehension problems. Laypersons whose knowledge was underestimated asked mainly for additional information previously not addressed in the explanations. The results suggest that underestimating a learner during the instructional dialog is as detrimental to learning as is the overestimation of a learner's knowledge. Thus, the provision of effective explanations presupposes an accurate mental model of the learner's knowledge prerequisites.